Providing an alternative means of user input, digitizer systems are commonly found in an ever increasing variety of computer applications. Typically, a sensing array is responsive to a stylus used by a user to enter data directly upon the array. The placement and/or movement of the stylus in pointing, writing and or sketching upon the array is used to control various computer functions.
In some embodiments the digitizer is provided directly over the visual display monitor (a touch screen), whereas in other embodiments the digitizer is provided as a separate and apart from the display (a touch pad). Use and application of either may be employed in various types of computer systems including laptops, home systems, kiosk, or other system displays.
Several types of stylus input digitizing devices are known in the prior art. One type involves direct contact of a stylus tip against a capacitive-resistive array. When the tip of the stylus is placed proximate to an area of the digitizing surface, a capacitive-resistive circuit within the pad detects the placement of the stylus and computes its location according to well-known mathematical formulas associated with grid-based arrays.
Other types of prior art system use an RF transmitter to send signals from the digitizer to the stylus or vis-a-versa, or may use a light source (visible or infrared) within the tip of the stylus which is directed by the user upon desired portions of the digitizer pad. In such cases the location of the stylus is determined by a processor decoding information from RF or light receivers. Yet another type of prior art system uses a surface-acoustic wave (SAW) device. With SAW, two transmitters set up a surface acoustic wave on the surface of the digitizer. The position of a person's finger or other stylus is detected by the finger or stylus reflecting/disturbing the acoustic wave.
Another type of prior art system uses an electromagnetic digitizer. The input of data by the user is realized as the result of a magnetic field emanating from the user's stylus interacting with the magnetic field or fields of the digitizer. Typically a grid of intersecting lines produces a field that can be either actively or passively modified by the field emanating from the stylus.
Each of these systems has had some success, yet each also has limitations that make them difficult if not undesirable for use. For example, direct contact technologies are subject to scratches and wear during normal operation. Wire-grid electromagnetic technology is quite expensive to implement and often requires additional heavy magnetic shielding behind the wire array to shield the system from undesirable magnetic influence. Light and RF systems require sophisticated electronics both for the source and for the receiver. Additionally, each and every one of these systems requires continuous power be supplied to the digitizer array in order to maintain the affect of stylus interaction.
In an effort to overcome one or more of these disadvantages applicants have recently proposed the use of an array of magnetic memory cells (each cell having a sense layer, intermediate layer and pinned reference layer) to provide an improved digitizer. While indeed an improvement in some ways, use of pinned reference magnetic memory cells presents certain disadvantages.
Generally, the principle underlying the storage of data in a magnetic media is the ability to change, and/or reverse, the relative orientation of the magnetization of a storage data bit (i.e. the logic state of a “0” or a “1”). The coercivity of a material is the level of demagnetizing force that must be applied to a magnetic particle to reduce and/or reverse the magnetization of the particle. Generally speaking, the smaller the magnetic particle the higher it's coercivity.
A prior art magnetic memory cell may be a tunneling magneto-resistance memory cell (TMR), a giant magneto-resistance memory cell (GMR), or a colossal magneto-resistance memory cell (CMR). These types of magnetic memory are commonly referred to as magnetic tunnel junction memory (MTJ). A magnetic tunnel junction memory generally includes a sense layer (also called a storage layer, data layer or bit layer), a reference layer, and an intermediate layer between the sense layer and the reference layer. The sense layer, the reference layer, and the intermediate layer can be made from one or more layers of material.
The sense layer is usually a layer of magnetic material that stores a bit of data as an orientation of magnetization M2 that may be altered in response to the application of an external magnetic field or fields. More specifically, the orientation of magnetization M2 of the sense layer representing the logic state can be rotated (switched) from a first orientation representing a logic state of “0” to a second orientation, representing a logic state of “1”, and/or vice versa.
The reference layer is a layer of magnetic material in which an orientation of magnetization M1 is “pinned”, as in fixed, in a predetermined direction. Often several layers of magnetic material are required and function as one to effectuate a stable pinned reference layer. The direction is predetermined and established by microelectronic processing steps employed in the fabrication of the magnetic memory cell.
Typically, the logic state (a “0” or a “1”) of a magnetic memory cell depends on the relative orientations of magnetization in the sense layer and the reference layer. For example, when an electrical potential bias is applied across the sense layer and the reference layer in a MTJ, electrons migrate between the sense layer and the reference layer through the intermediate layer. The intermediate layer is typically a thin dielectric layer commonly referred to as a tunnel barrier layer. The phenomena that cause the migration of electrons through the barrier layer may be referred to as quantum mechanical tunneling or spin tunneling.
The logic state may be determined by measuring the resistance of the memory cell. For example, if the overall orientation of the magnetization in the sense layer is parallel to the pinned orientation of magnetization in the reference layer the magnetic memory cell will be in a state of low resistance. If the overall orientation of the magnetization in the sense layer is anti-parallel (opposite) to the pinned orientation of magnetization in the reference layer the magnetic memory cell will be in a state of high resistance.
In an ideal setting the orientation of the alterable magnetic field in the sense layer would be either parallel or anti-parallel with respect to the field of the reference layer. As the sense layer and the reference layer are generally both made from ferromagnetic materials and are positioned in close permanent proximity to each other, the generally stronger reference layer may affect the orientation of the sense layer. More specifically, the magnetization of the reference layer may generate a demagnetization field that extends from the reference layer into the sense layer.
The result of this demagnetization field from the reference layer is an offset in the coercive switching field. This offset can result in asymmetry in the switching characteristics of the bit: the amount of switching field needed to switch the bit from parallel to anti-parallel state is different from the switching field needed to switch the bit from anti-parallel state to parallel state. To have reliable switching characteristics and to simplify the read/write circuitry, it is desirable to have this offset reduced to as near zero as possible.
The magneto-resistance ΔR/R may be described as akin to a signal-to-noise ratio S/N. A higher S/N results in a stronger signal that can be sensed to determine the state of the bit in the sense layer. Thus, at least one disadvantage of a tunnel junction memory cell having a pinned reference layer in close and fixed proximity to the sense layer is a potential reduction in the magneto-resistance ΔR/R resulting from the angular displacement.
To pin the reference layer during manufacturing, the reference layer must be heated to an elevated temperature in an annealing step. The annealing step typically takes time, perhaps an hour or more. As the reference layer is but one part of the memory being produced, the entire memory must be subject to temperatures ranging from about 100 to 300 degrees centigrade while under the influence of a constant and focused magnetic field. Such manufacturing stresses may permit the reference layer to become un-pinned and lose it's set orientation if the memory is later subjected to high temperatures. In addition, the characteristics of the sense layer may be unknowingly affected by heat during some manufacturing processes.
To facilitate establishing a pinned reference layer it is not uncommon for the reference layer to include multiple layers of material. While utilizing multiple layers may help ensure that the reference layer remains pinned, it also raises the complexity of manufacturing each and every memory cell present in the magnetic memory.
When employed in a digitizer array, the magnetic memory cells are initialized such that each sense layer is oriented in a predetermined direction. As the users moves the stylus across the array, the magnetic field emanating from the stylus re-orients the sense layers most proximate to the stylus. The user directed re-orientation is registered by the system when the digitizer array is scanned. To register the user's next movement of the stylus, the digitizer array is re-initialized. As the scanning and re-initializing are performed every few microseconds the movements of the stylus by the user are recognized to the system.
Because of the ever present magnetic field of the pinned reference layer, the coercivity of the sense layer must at a minimum be greater than the offset in the coercive switching field described above. More specifically, the minimum coercivity of the sense layer is dictated at least in part by the offset force created by the pinned reference layer. In addition, as the magnetic field of the reference layer is constant, the coercivity of the sense layer must be great enough that the sense layer will maintain orientation at least until the system cycles through a read scan operation. Such longevity of orientation and heightened level of coercivity directly require higher levels of current to pass through the digitizer array. As such the size of the power supply and current conductors must be larger than might otherwise be desired as an issue of space and manufacturing cost.
As noted above, digitizers may exist as touch pads connected to a removed display or as touch screens that immediately display information beneath the stylus. The disadvantages noted above are present as well in such touch screen displays. Additionally, as a touch screen application requires both the elements of the display and the elements of the digitizer to be proximately located, the issues of size and space between components are even greater.
Hence, there is a need for an improved digitizing magnetic memory cell array that overcomes one or more of the drawbacks identified above. The present invention satisfies one or more of these needs.